Litterfall mercury reduction on a subtropical evergreen broadleaf forest floor revealed by multi-element isotopes

Litterfall mercury (Hg) deposition is the dominant source of soil Hg in forests. Identifying reduction processes and tracking the fate of legacy Hg on forest floor are challenging tasks. Interplays between isotopes of carbon (C) and nitrogen (N) may shed some lights on Hg biogeochemical processes because their biogeochemical cycling closely links with organic matters. Isotope measurements at the evergreen broadleaf forest floor at Mt. Ailao (Mountain Ailao) display that δ²⁰²Hg and Δ¹⁹⁹Hg both significantly correlate with δ¹³C and δ¹⁵N in soil profiles. Data analysis results show that microbial reduction is the dominant process for the distinct δ²⁰²Hg shift (up to ∼1.0‰) between Oi and 0-10 cm surface mineral soil, and dark abiotic organic matter reduction is the main cause for the Δ¹⁹⁹Hg shift (∼-0.18‰). Higher N in foliage leads to greater Hg concentration, and Hg⁰ re-emission via microbial reduction on forest floor is likely linked to N release and immobilization on forest floor. We thus suggest that the enhanced N deposition in global forest ecosystems can potentially influence Hg uptake by vegetation and litter Hg sequestration on forest floor.